[63.01] Impact Induced Climate Change on Venus: The Role of Large Comets

D.H. Grinspoon, M.A. Bullock (Southwest Research Institute)

The surface temperature of Venus is a sensitive function of
the abundances of greenhouse gases and also of cloud
structure. In previous work we have studied the climate
impact of past and continued outgassing of greenhouse and
cloud-forming gases (1) and tectonic signatures that may
have resulted from volcanically induced climate change (2).
These studies showed that in outgassing events where large
amounts of both H2O and SO2 are released, the
increased albedo that arises from thickening of the clouds
can, to some extent, ameliorate the greenhouse warming
expected from increased abundances of these IR absorbing
gases. The largest warming typically arises several hundred
million years after an outgassing event when most of the
excess SO2 has been removed by reaction with surface
minerals, but much of the atmospheric H2O remains
(because it is removed by exospheric escape on longer time
scales). This combination - enhanced H2O abundance with
SO2 returned to 'normal' - leads to maximum warming
because the cloud thickness, and thus the albedo, is limited
by the availability of SO2, whereas IR absorption in
CO2 windows by enhanced H2O can cause warming on the
order of 100 K. It seems likely that large comet impacts
should also produce such a situation. The atmosphere of
Venus currently contains 7 x 1018 grams of water, about
as much as in a 25 km diameter comet. Comets may have been
an important contributor to the current water inventory on
Venus. Much of this may have been supplied by a few large
comet impacts in the last several hundred million years (3).
We will report on new runs of our Venus Evolutionary Climate
Model which simulate the volatile input from large comet
impacts and investigate the climate effects of these events.
Calculation will be done with cometary delivery alone, and
in conjunction with various outgassing scenarios. This
allows us to examine how the vulnerability of the Venusian
climate system to impact induced climate change is affected
by the relative timing of large magmatic and impact events.
(1) Bullock, M.A., and D.H. Grinspoon, J. Geophys. Res. 101,
7521-7529, 1996. (2) Solomon, S.C., M. A. Bullock, and D. H.
Grinspoon, Science, 286: 87-90, 1999. (3) Grinspoon, D.H.
and J.S. Lewis, Icarus, 74, 21-35, 1988.